Previous work in this laboratory has shown that dramatic changes in the fluidity of the plasma membrane of chick embryo heart cells occur during embryonic development and during post-hatching maturation. Changes in the lipid composition of the plasma membrane appear to be partly responsible for the changes in fluidity that occur. Previous work has also shown that marked changes in the activities of various membrane transport systems occur during development and that the permeability of the plasma membrane to substances that are taken up by simple diffusion also changes during development. The purpose of the proposed research is to determine whether the changes in membrane transport functions that occur during development are caused by the changes in membrane fluidity. The general question we are asking is: is there a relationship between plasma membrane fluidity and the function of membrane transport systems? We will begin with studies on red blood cells because the plasma membrane of the red cell is the best characterized preparation with regard to transport properties, membrane structure and composition, and membrane fluidity. We intend to alter the fluidity of the plasma membrane by incubating red cells with liposomes or by treatment with local anesthetics. Incubation with lecithin liposomes will deplete the membranes of cholesterol and increase fluidity. Incubation with liposomes containing equimolar amounts of cholesterol and lecithin will increase the cholesterol level of the membranes and decrease fluidity. Treatment with local anesthetics will increase fluidity. The fluidity of the treated cells will be estimated from the depolarization of the fluorescence of 1,6-diphenylhexatriene. In the same cells we will determine the activity of the Na,K-ATPase (an active transport system), the level of glucose transport (by a facilitated transport system), and the permeability to erythritol (which enters by simple diffusion). Similar experiments will be subsequently performed with chick embryo heart at various stages of development.